This application claims the priority of German application No. 100 38 741.1, filed Aug. 9, 2000, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a particular method and a particular system for feeding a reducing agent into a catalyst device.
In addition to carbon monoxide (CO) and hydrocarbons (HC), nitrogen oxides (NOx), in particular, are among the directly emitted primary pollutants which endanger the environment and which are generated during operation of internal-combustion engines, particularly diesel engines. It is not possible to use three-way catalysts, such as those used in xcex=1 controlled Otto engines and gas engines, because of excessive oxygen in diesel engine exhaust gas. In order to reduce nitrogen oxide emissions in diesel engines, a selectively operating SCR-catalyst (SCR=Selective Catalytic Reduction) has therefore been developed. In this catalyst, the expelled nitrogen oxides are reduced to N2 and H2O by a fed reducing agent, specifically ammonia (NH3).
This manner of reducing nitrogen oxide emissions was found to be successful in stationary diesel engines. In these stationary systems, an SCR-catalyst is arranged in the exhaust gas train of the internal-combustion engine. The NH3 is admixed to the exhaust gas flow in front of the SCR-catalyst by nozzle injection. In such systems, the NH3 is fed as gas or as an aqueous solution. When aqueous NH3 solutions are fed, thermohydrolytic splitting takes place in the exhaust gas flow or in the SCR-catalyst in order to set free the NH3 required to reduce the nitrogen oxides. The reducing agent is fed by way of a metering device which is adjusted as a function of the expected NOx quantity in the exhaust gas flow of the diesel generator.
The need for caution in handling NH3 as a gas or as an aqueous solution led to the approach of providing the NH3 required for the NOx by feeding aqueous urea solution into the exhaust gas flow. The heat of the exhaust gas flow or of the catalyst will bring about thermohydrolytic splitting under exhaust gas of NH3 as the reducing agent.
A system is known from German Patent Document DE 297 08 591 for feeding ammonia in the exhaust gas flow of an internal-combustion engine.
A method is known from German Patent Document DE 43 15 278 A1 for metering a reducing agent into a nitrogen-oxide-containing exhaust gas of an internal-combustion engine having a catalyst for reducing nitrogen oxides which is installed in an exhaust pipe. In this known method, the rate at which the reducing agent is charged into the exhaust gas is adjusted as a function of operation-relevant parameters of the exhaust gas, of the catalyst, and, optionally, of the engine.
The reducing agent, in the form of an aqueous urea solution, is stored in liquid form in a reducing-agent tank and is fed by way of a reducing-agent feed pipe to a reducing agent injection valve. The liquid reducing agent is injected by way of this reducing-agent injection valve in front of the catalyst.
One disadvantage of this known exhaust gas after treatment method is that, because of the storage and admixing of aqueous urea solution, a large mass or a large volume of the reducing agent must be carried along. This is necessary so that no refuelling with reducing agent by the customer will be required during a reasonable maintenance interval of, for example, 30,000 kilometers.
It is an object of the present invention to provide a method and a system for feeding a reducing agent into a catalyst device which require a smaller mass or a smaller volume of reducing agent to be carried along.
According to the invention, a reducing agent is fed into a catalyst device by providing a first reservoir having a solid reducing agent, providing a second reservoir having a solvent for the solid reducing agent, producing a reducing agent solution from the solid reducing agent and the solvent, intermediately storing the reducing agent solution in a third reservoir, and feeding the reducing agent solution from the third reservoir into the catalyst device.
The invention combines the advantages of a solid reducing agent, such as urea or other soluble substances which set free NH3, with the simple apportioning of a liquid reducing agent.
A solid reducing agent, for example, in the form of powder, pressed parts, or monoliths, is carried along onboard. Furthermore, a corresponding solvent is either carried along in a separate tank or segregated from existing sources. An intermediate storage device is expediently provided. The reducing agent is present as a liquid solution in the intermediate storage device. The solid reducing agent and the solvent are mixed with one another by a suitable metering or mixing system in order to obtain a defined concentration. Then, as is known, the liquid reducing-agent solution is supplied to a pump and an injection valve so that the solution is metered in a controlled or regulated manner into the exhaust gas train. The customer must resupply water only if no rain water or condensate from an air-conditioner compressor or the like is available.
Advantageous further developments and improvements of the invention are reflected in various claims.
According to one preferred feature of the invention, the reducing-agent solution is produced by metered introduction of the solid reducing agent and the solvent into the third reservoir.
According to another preferred feature, the reducing-agent solution is produced by the metered introduction of the solid reducing agent and of the solvent into a mixing device provided between the first reservoir and the third reservoir. In this case, the reducing-agent solution is conducted from the mixing device into the third reservoir.
According to another preferred feature, the concentration of the reducing-agent solution is measured, and the metered introduction is controlled such that a predeterminable concentration is adjusted.
According to another preferred feature, the solid reducing agent is pulverized.
According to another preferred feature, the solid reducing agent is made available in prepressed units of mass.
According to another preferred feature, the solid reducing agent is provided as an extruded basic body.
According to another preferred feature, the basic body is dipped into the solvent or the reducing-agent solution to produce the reducing-agent solution.
According to still another preferred feature, the dipping time is determined as a function of a desired or detected concentration.
According to yet another preferred feature, the solvent is water obtained by condensation.